There can be a "die off" period, according to what I've read, when drastically switching diets. Certain bacteria that survived off of meat etc now die off and are replaced with new bacteria. I'm sure google will give you some info on vegetarian diets and UC. This article outlines a semi-vegetarian diet for UC and includes some research.
/www.dietvsdisease.org/ulcerative-colitis-diet/For years, a low fiber diet has been recommended by doctors for patients who are flaring but it is not based on any science, just patient reported symptoms. Many people will report fiber upsetting their bowels during a flare but this is all individual. All the latest research shows fiber being good for UC, in general. For me, eating complex carbs and lactose kills me during a flare because of gas. Easily digested vegetables don't bother me at all.
What exactly are you eating?